1. Field of the Invention
The present invention relates to a charge pump circuit; in particular, the present invention relates to a charge pump circuit with current detecting and a feedback method thereof.
2. Description of Related Art
Light Emitting Diodes (LED's) provide features of, among others, high luminance efficiency, high brightness, low power consumption, long lifespan, reliable security, environment friendliness and fast startup, and numerous high-technology products are now adopting LED's as their light sources for applications. In general, LED driving is commonly accomplished by means of a current source offering constant direct current, thereby maintaining the stable brightness thereof. Especially, for portable devices using batteries as their power sources, since such batteries may be merely able to provide low voltage power, they are not suitable for driving the light source composed of multiple LED's connected in series. This means, it is required to boost the voltage of the batteries so as to successfully drive these LED's. Typically the boost DC-DC converter which is utilized to increase battery voltages, provides high conversion efficiency, but undesirably leads to significantly increase in manufacturing cost and PCB area occupation as well. Another approach for boosting battery voltages is to use a charge pump circuit.
The most concise method for the LED brightness adjustment according to the prior art is to adjust the LED current. FIG. 1 shows a current variation diagram in accordance with the LED brightness adjustment according to the prior art. In a conventional method, it adjusts the current I1, higher brightness, to the current I2, lower brightness, whereas in this way drifts of the LED emission spectrum may occur. For certain high-leveled products, or devices particularly sensitive to luminance chromaticity, e.g., monitors, such drifts of the LED emission spectrum can not be tolerated.
To eliminate the problem caused by the LED emission spectrum drifts, another technology is developed which applies a characteristic of human eyes, a persistence of vision, for flash light to control and rapidly switch the current inputted to the LED, thereby resulting in the LED twinkling effects. FIG. 2 shows a current switching diagram in accordance with the LED brightness adjustment according to the prior art. It is possible to use a switch device to quickly switch the current I1 shown in FIG. 1. Because of the characteristic of human eyes, a persistence of vision, for flash light, especially regarding to the duration of flash light which is less than 16.67 μs, i.e.: switched at 60 Hz or more in frequency, the flash light becomes undistinguishable. By using such a technology, therefore, it is effectively equivalent to generate the luminance by supplying the average current to the LED without causing LED emission spectrum drifts.
The conventional charge pump circuit only comprises a few components such as switches and capacitors, and upon operation of the switches in the charge pump circuit to charge the capacitors, without the existence of resistors or others components for current blockage, the input current may generate current surges as shown in FIG. 3. Hence, when the switch is on and starts to charge the capacitor, the input current initially charges with greater current, but then the input current reduces as the voltage of the capacitor increases. Since the portable device usually employs batteries as its power source, sudden output of massive current may lead to dramatic falloff in the battery voltage. Additionally, the portable device is commonly equipped with several circuits of different functions therein at the same time, all of which use this power source to execute. As a result, other circuits may be inevitably interfered by the surges shown in FIG. 3; for example, noises may be generated in an audio circuit and the like.
Accordingly, in order to achieve the objectives of providing stable output current to lighting devices such as LED's and zero interference to other co-existing circuits in the portable device using the same power source, it is necessary to make further improvements in accordance with the charge pump circuit according to the prior art.